VPD values!! How much off the recommended ranges is not good anymore??

Depending on the chart you look at, 1.5 may be great for a mature plant or it might be considered too high... early on for seedlings and such this is probably too much but they'll still grow fairly healthy if not as fast. there are ways to mitigate high VPD, like lowering concentration of nutes, since water uptake is increased you can still give it the same rate of nutes by reducing concentration. Anyway, while the concepts behind this are rock solid, i don't beleive there is a lot of reseaarch to fully flesh it out, especially specific research on marijuana plants and any variances it may have to the general reaserch down on other plants on this subject. So, as always ppl take a concept and run with it.... and not always with any evidence to back up what they say.. So, use the charts, but always let plant observations be the last say. If you cannot control the temps and RH, knowing VPD will limit it's usefulness... liek i said above you can react to high/low vpd by changing nute concentration in an ateempt to maintain rate of what the plant takes in. roots are not selective beyond what fits through semipermeable memebranes.. though there is some active diffussion for some molecules occuring too. otherwise it's just factors of osmosis.. conecntration gradients dictate flow. Think stomatas close in darkness? so the vpd at night may not be suprememly relevant or at least not as important as light hours (might want to google how stomata function just to be sure) i have a chart that shows .5-1.2 being in teh "green". i've seen others that show higher rnages like you mention... they both are doing the math correctly and look professional, but i doubt either has done any proper reseaarch with a proper baseline comparison. It's good science but i highly doubt anyone can be certain about what is "optimal" for the marijana plant species and genetics will likely vary quite a bit due to region the plant comes from or the mish-mash of genetics from multiple regions... consider it a good ballpark to start in and further refine through observing how the plant reacts... just like anything else we do. what's? too much? start approaching 2.0+ and growth slows in my exp with some vege plants last winter... when i reduced concentration of fertilizer (soilles, provided each time), i had a good reaction under bad VPD conditions. My RH was below 30%.. temps were high... it was high VPD and yield suffered due to slow growth and alotted time (schedule)